Exhaust gas purification for a hydrogen engine

ABSTRACT

An exhaust gas purification system for a hydrogen engine is provided in which hydrogen gas is supplied in the hydrogen engine as a fuel for combustion, and is also supplied to the exhaust gas after combustion to improve purification of the exhaust element. The exhaust gas purification system includes a hydrogen storage tank, the hydrogen engine, and a catalyst provided in an exhaust pipe, and is capable of supplying hydrogen gas through the exhaust pipe during operation of the hydrogen engine. An upstream-side exhaust gas sensor is provided on the exhaust pipe upstream of the catalyst. An exhaust-side hydrogen gas injection device is provided upstream of the upstream-side exhaust gas sensor. Control means control the amount of hydrogen gas for supply to the hydrogen engine based on the detection by the upstream-side exhaust gas sensor as a feedback control so as to obtain the exhaust gas having a predetermined lean air-fuel ratio, and control the amount of hydrogen gas supplied from the exhaust-side hydrogen gas injection device to be less than that supplied to the hydrogen engine.

TECHNICAL FIELD

This invention relates to exhaust gas purification systems for a hydrogen engine, and more particularly to an exhaust gas purification system for an hydrogen engine which allows purification, with hydrogen gases, of particular components contained, after combustion in the hydrogen engine, in the exhaust gas.

BACKGROUND

While electric vehicles equipped with a fuel cell as a power source of the vehicle have received attention recently, some attention has also been drawn to hydrogen engine which create driving power by direct combustion of hydrogen acting as a fuel in the engine.

A hydrogen engine includes a hydrogen storage lank containing hydrogen at a high pressure and creates the driving force for the vehicle by combustion of hydrogen supplied from the hydrogen storage tank.

Taking advantage of the pressure of hydrogen stored in the tank at high pressure, hydrogen forming the fuel supply is directly injected into a combustion chamber in the hydrogen engine.

PRIOR ART

The following documents 1 to 7 form the prior art to the present patent application;

Document 1: Japanese Patent No. 3661555

Document 2: Japanese Patent Application Publication No. H02-86915

Document 3: Japanese Patent Application Publication No. 1102-149714

Document 4: Japanese Patent Application Publication No. 2002-13412

Document 5: Japanese Patent Application Publication No. 2005-240656

Document 6: Japanese Utility Model Application Publication No. H05-13937

Document 7: Japanese Utility Model Application Publication No. H06-10129

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In conventional exhaust gas purification systems for hydrogen engines, a problem results from the fact that harmful ammonia (NH₃) is generated during operation of the hydrogen engine when the excessive air ratio λ of the mixture (in other words the “air-fuel ratio”) is less than 1 (λ<1) and the ratio of nitrogen in the air is reduced due to excessive hydrogen (“rich”) during combustion of hydrogen gas.

This is expressed simply as a chemical formula:

H₂+O₂+N₂→H₂O+NH₃

A problem also arises through the fact that a large amount of nitrogen oxides (NO_(x)), which are harmful components contained in the exhaust gas, is emitted when the air-fuel ratio λ of the mixture is equal to one or more (λ≧1).

This is expressed simply as a chemical formula:

H₂+O₂+N₂→H₂O+NO_(x)

In a control of the air-fuel ratio λ using the aforementioned exhaust gas purification system, if the air-fuel ratio λ of the air-fuel mixture is at the theoretical air-fuel ratio, it should be equal to one (λ=1). If the air-fuel ratio λ is one (λ=1), the theoretical air-fuel ratio (A/F) should be equal to 34.3 (A/F=34.3) when the fuel employed is hydrogen gas.

Further, in case the combustion gas does not contain carbon monoxide (CO) or hydrocarbons (HC) in the exhaust gas as a reducing agent, a problem results from the fact that the catalysts which are widely used for exhaust gas systems of vehicles, cannot be utilized as a post-purification system.

In such a case, a lean NO_(x) catalyst is in practical use for NO_(x) purification system, this having poor purification performance and heat resistance.

Moreover, in a urea charging NO_(x) reducing system, which is utilized on a large-sized diesel vehicle, a problem also exists resulting from the fact that additional devices for supplying urea to the exhaust system are required.

The object of the present invention is to provide an exhaust gas purification system for a hydrogen engine to improve exhaust purification performance in a manner such that stored hydrogen gas is combusted as a fuel in the hydrogen engine and is also supplied to the exhaust gas after combustion.

In addition, during primary supply to the hydrogen engine for combustion, hydrogen gas is combusted so as to restrict emission of ammonia.

Hydrogen gas is secondarily supplied to the exhaust gas to improve the purification performance for a particular component (nitrogen oxide or NO_(x)) in (he exhaust gas that increases with the restriction of emission of ammonia.

Further, the present invention reduces consumption of hydrogen gas utilized for maintaining (he purification performance.

Thanks to the features mentioned above, fuel economy is improved.

Means to Solve the Problems

In order to obviate the above-mentioned inconveniences, the present invention provides an exhaust gas purification system for a hydrogen engine, having a hydrogen storage tank for storing hydrogen at high pressure, a hydrogen engine for combusting hydrogen supplied from the hydrogen storage tank, and a catalyst provided in the exhaust pipe of the hydrogen engine. The exhaust gas purification system is capable of supplying hydrogen gas through the exhaust pipe during operation of the hydrogen engine. An upstream-side exhaust gas sensor is provided in the exhaust pipe upstream of the catalyst for detecting the exhaust element. An exhaust-side hydrogen gas injection device is provided upstream of the upstream-side exhaust gas sensor for injecting hydrogen gas into the exhaust pipe. A control means controls the amount of hydrogen gas for supplying to the hydrogen engine based on detection at the upstream-side exhaust gas sensor as a feedback control so as to obtain exhaust gas having a predetermined lean air-fuel ratio, and controls the amount of hydrogen gas supplied from the exhaust-side hydrogen gas injection device to be less than that supplied to the hydrogen engine.

Effects of the Invention

As thus described the present invention provides a exhaust gas purification system for a hydrogen engine, having a hydrogen storage tank for storing hydrogen at high pressure, a hydrogen engine for combusting hydrogen supplied from the hydrogen storage tank, and a catalyst provided in the exhaust pipe of the hydrogen engine. Exhaust gas purification systems are capable of supplying hydrogen gas through the exhaust pipe during operation of the hydrogen engine. An upstream-side exhaust gas sensor is provided in the exhaust pipe upstream of the catalyst for detecting the exhaust composition. An exhaust-side hydrogen gas injection device is provided upstream of the upstream-side exhaust gas sensor for injecting hydrogen gas into the exhaust pipe. Control means control the amount of hydrogen gas supplied to the hydrogen engine based on the detection by the upstream-side exhaust gas sensor as a feedback control so as to obtain an exhaust gas having a predetermined lean air-fuel ratio, and controls the amount of hydrogen gas supplied from the exhaust-side hydrogen gas injection device in order to be less than that supplied to the hydrogen engine.

Accordingly, the combustion of the gas is maintained on average in the lean mixture state, which considerably reduces the emission from the exhaust system, in particular NH₃.

Resultant increasing NO_(x) can be selectively reduced on the catalyst with the addition of the secondarily supplied hydrogen gas, thereby maintaining an overall high level of purification performance of the catalyst.

Further, the consumption of the supplied secondary hydrogen gas can be decreased.

BEST MODE FOR CARRYING OUT THE INVENTION

By such invention, the control means control the amount of hydrogen gas supplied to the hydrogen engine based on the detection of the upstream-side exhaust gas sensor as a feedback control with a result that that the exhaust gas has a predetermined lean air-fuel ratio.

Also the control means control the amount of hydrogen gas supplied from the exhaust-side hydrogen gas injection device so that it is less than that supplied to the hydrogen engine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-6 illustrate the embodiment of the present invention.

An embodiment of the present invention is described below with reference to the drawings.

FIG. 1 is a flow-chart for an exhaust gas purification system for a hydrogen engine according to an embodiment of the present invention.

FIG. 2 is a schematic diagram for the exhaust gas purification system of the hydrogen engine.

FIG. 3 is a schematic enlarged cross-sectional view of the hydrogen engine showing a slightly stratified charge type.

FIG. 4 is a diagram showing the distribution of the mixture when viewing a combustion chamber from a piston.

FIG. 5 is a diagram showing fuel injection timing upon starting of the engine in the exhaust gas purification system of the hydrogen engine.

FIG. 6 is a diagram showing fuel injection timing after start of the engine in the exhaust gas purification system of the hydrogen engine.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The exhaust gas purification system 1 includes: a hydrogen storage tank 2 for storing hydrogen at high pressure (several tens of MPa, for example 35-70 MPa); the hydrogen engine 3 for combusting hydrogen supplied from a hydrogen storage tank 2; and a catalyst 5 disposed in an exhaust pipe 4 of the hydrogen engine 3. Hydrogen gas can be supplied in trace amounts to the exhaust pipe 4 during operation of the hydrogen engine 3.

Hydrogen engine 3 is designed for a cylinder injection type of fuel supply, and is a four-stroke engine equipped with a turbocharger 23 mentioned later.

As shown in FIGS. 2 and 3, (he exhaust gas purification system 1 for the hydrogen engine 3 includes a cylinder block 6, a cylinder head 7, and a cylinder head cover 8.

In cylinder block 6, a piston 9 is slidably mounted. Also a combustion chamber 10 is formed in cooperation with the cylinder head 7.

Further, cylinder head 7 includes, in an intake system, an intake camshaft 11, and an intake valve 12 driven by the intake camshaft 11.

The cylinder head 7 also includes, in the exhaust system, an exhaust camshaft 13 and an exhaust valve 14 driven by the exhaust camshaft 13.

The intake system of the hydrogen engine 3 is connected in the following order, to: an air cleaner 15; an intake pipe 16 for introducing the intake air from the air cleaner 15 toward the hydrogen engine 3; a throttle body 18 equipped with a throttle valve 17; and an intake manifold 20 integrating a surge tank 19 and attached to the cylinder head 7.

The exhaust system of the hydrogen engine 3 is connected in the following order, to: an exhaust manifold 21 attached to the cylinder head 7 for the passage of the exhaust gas from the hydrogen engine 3; and an exhaust pipe 4 having an upstream side connected to the exhaust manifold 21 and including a catalyst converter 22 that contains the catalyst 5.

Also, the hydrogen engine 3 is equipped with a turbocharger 23 for compressing the intake air from the air cleaner 15 and supplying compressed air to the hydrogen engine 3.

Turbocharger 23 includes in a turbocharger case 24: a compressor 25 disposed in the intake pipe 16; and a turbine 26 disposed in the exhaust pipe 4 between an intake manifold 21 and the catalyst converter 22 so as to be rotated by the flow of exhaust gases. The flow of exhaust gases to the turbine 26 is adjusted by a wastegate mechanism 28 having a wastegate valve (or “wastegate control VSV”) 27.

Intake pipe 16 between the compressor 25 and the throttle body 18 is equipped an intercooler 29 for cooling (he intake air compressed by the turbocharger 23.

Hydrogen engine 3 is provided with a hydrogen gas injection system 30 for injecting hydrogen gases.

This hydrogen gas injection system 30 includes an intake-side hydrogen gas injection device 31 and an exhaust-side hydrogen gas injection device 32.

In the intake system of the hydrogen engine 3, the intake-side hydrogen gas injection device 31 is a cylinder injection type which directly injects hydrogen gases into the combustion chamber 10.

The exhaust-side hydrogen gas injection device 32 injects hydrogen gases into the exhaust pipe 4.

Hydrogen gas injection system 30 includes the hydrogen storage tank 2, a hydrogen gas supply passage 33 having one end connected to the hydrogen storage tank 2 and the other end connected to a pressure regulator 34.

Pressure regulator 34 has a two-way switching function and a decompressing function for decompressing hydrogen gas stored in the storage tank 2 at high pressure (several lens of MPa, for example 35-70 MPa) to a lower pressure (several hundreds kPa, e.g. several atmospheric-pressures).

Intake-side hydrogen gas injection device 31 includes a primary supply passage 35 having one end connected to the pressure regulator 34 and the other end connected to a delivery pipe 36 associated with the cylinder head 7, and a primary supply injector (or inside-cylinder injector) 37.

Exhaust-side hydrogen gas injection device 32 includes a secondary supply passage 38 having one end connected to the pressure regulator 34 and the other end connected to a secondary supply injector 39 for injecting hydrogen gas into the exhaust pipe 4.

Hydrogen engine 3 is also equipped with an idle speed controller 40.

Idle speed controller 40 includes a bypass passage 41 for communicating the throttle body 18 and the surge tank 19 while bypassing the throttle valve 17, and an ISC valve (or idling air quantity control valve) 42 disposed in the bypass passage 41 for regulating the flow rate of idle air to the hydrogen engine 3.

Further, the cylinder head cover 8 of the hydrogen engine 3 is provided with an ignition coil 43 and a PCV valve 44.

To the PCV valve 44, a tank-side blowby gas passage 45 is connected for communicating to the surge tank 19.

To the cylinder head cover 8, a cleaner-side blowby gas passage 46 is connected for communicating to the air cleaner 15.

Hydrogen engine 3 is equipped with a fuel pressure sensor 47, and an engine coolant temperature sensor 49.

Fuel pressure sensor 47 is attached to the delivery pipe 36 to detect the pressure of the fuel delivered to the primary supply injector 37.

Engine coolant temperature sensor 49 detects the coolant temperature in a coolant passage 48 formed in a part of the hydrogen engine 3.

Throttle body 18 includes a throttle sensor 50 for detecting the opening angle of the throttle valve 17 and is connected to one end of a pressure introducing passage 51 downstream from the throttle valve 17.

At the other end of the pressure introducing passage 51, an intake pressure sensor 52 is connected for detecting the intake pipe pressure downstream from the throttle valve 17.

Surge tank 19 is provided with an intake temperature sensor 53 for detecting the temperature of the intake air.

On the upstream side of the catalyst converter 22, that is in the exhaust pipe 4 upstream of the catalyst 5, an upstream-side exhaust gas sensor (referred to as “air-fuel ratio sensor”, or “λ sensor”) 54 is disposed for detection in the exhaust system. On the upstream side of the upstream-side exhaust gas sensor 54, the exhaust-side hydrogen gas injection device 32 for injecting hydrogen gas into the exhaust pipe 4 is disposed.

Also, the exhaust pipe 4 downstream from the catalyst 5 is provided with a downstream-side exhaust gas sensor (Hydrogen (H2) sensor or NOx sensor) 55 for detecting the exhaust element.

As shown in FIG. 2, the hydrogen engine 3 includes on the exhaust side: the exhaust manifold 21, the turbine 26 of the turbocharger 23, the upstream-side exhaust gas sensor 54, the catalyst converter 22, and the downstream-side exhaust gas sensor 55 in this order from the upstream side. Secondary supply injector 39 of the exhaust-side hydrogen gas injection device 32 is disposed at a downstream end of the exhaust manifold 21 upstream of the turbine 26 of the turbocharger 23 so that the exhaust side hydrogen gas injection device 32 injects hydrogen gases into the exhaust pipe 4.

A control means (ECM) 56 is connected with the wastegate valve 27, (he pressure regulator 34, the primary supply injector 37 of the intake-side hydrogen gas injection device 31, the secondary supply injector 39 of the exhaust-side hydrogen gas injection device 32, the ISC valve 42, the ignition coil 43, the fuel pressure sensor 47, the engine coolant temperature sensor 49, (he throttle sensor 50, the intake pressure sensor 52, the intake temperature sensor 53, the upstream-side exhaust gas sensor 54, and the downstream-side exhaust gas sensor 55.

Control means 56 is connected with a crank angle sensor 57, and a battery 60 through a main switch 58 and a fuse 59.

Crank angle sensor 57 detects the angle of a crank to permit the control means 56 to determine start timing for fuel injection.

Hydrogen engine 3 of the cylinder injection type for fuel supply is described below in detail.

The cylinder injection system is preferable for gaseous fuel (such as hydrogen gas) to prevent emission of gases when (he vehicle stops and icing of the injector.

Such cylinder injection system achieves high power, better fuel economy, and low emission owing to the direct injection of hydrogen gas at high pressure into the combustion chamber 10.

Also as shown in FIG. 3, a tumble flow is generated owing to (he straight shape of an intake port 61 of the hydrogen engine 3 and the shape of the top surface of the piston 9. By exact control of the timing of fuel injection, a rich mixture layer is produced adjacent an ignition plug 62 for stable combustion irrespective of a cold or warmed engine, which is a slightly stratified charge type.

As shown in FIG. 3, the slightly stratified charge type adopts the intake port 61 as the straight port extending straight to the intake valve 12 for accelerating the flow of the intake air. An oval concave section 63 offset at the top of the piston 9 generates the tumble flow (upward stream).

At this moment, the fuel is injected at an appropriate angle under high pressure in an intake stroke to the combustion chamber 10.

Fuel injected into the combustion chamber 10 in the intake stroke is guided by (he tumble flow, as shown in FIG. 4, to form a distribution of the mixture in which the rich mixture is formed adjacent the ignition plug 62 and lean mixture adjacent the outer circumference. The overall combustion chamber 10 is controlled to achieve the theoretical air-fuel ratio for stable combustion.

Also, in the fuel injection control of the hydrogen engine 3 adapted to the cylinder injection system, the timing and duration (in other words, the amount) of the fuel to be injected from the primary supply injector 37 are controlled to inject the fuel in the optimum amount at the optimum timing.

The timing and duration of the fuel injection are determined by a fuel injection control performed upon starting the engine and a fuel injection control performed after the start of the engine during the ordinary running of the vehicle.

For protection of the engine and improvement of fuel economy, a fuel-cut control is performed according to driving states of the vehicle.

The aforementioned fuel injection control upon starting the engine is explained below.

FIG. 5 illustrates the fuel injection timing determined by the fuel injection control upon starting of the engine. When the engine speed is below a predetermined rotational speed, the injection is carried out sequentially at injection timing (see hatched parts designated by letter “A” in FIG. 5) based on a signal from the crank angle sensor.

At a very low temperature, divisional injections are performed for several times at a certain interval after rise and reception of the crank angle sensor signal.

In addition, (he duration of fuel injection by the fuel injection control upon starting of the engine is estimated by determining a basic fuel injection duration upon starting of the engine based on the coolant temperature and correcting this basic duration by the engine speed, voltage, and the like.

Basic duration upon starting of the engine is increased with the decrease of coolant temperature to improve the startability of the engine.

The aforementioned fuel injection control after start of the engine is explained below.

Fuel injection control after start of the engine includes synchronized injection and asynchronous injection.

As shown in FIG. 6, the synchronized injection is performed sequentially in a normal state at the injection timing based on the signal from the crank angle sensor (see hatched parts designated by letter “A” in FIG. 6).

During deceleration of the vehicle, upon canceling of the fuel-cut, and during acceleration of the vehicle, the asynchronous injection does not synchronize with the signal from the crank angle sensor; fuel injection is performed at the same time for all of the cylinders (see bold hatched parts designated by letter “B” in FIG. 6).

Further, duration of fuel injection by the fuel injection control after the start of the engine is established by determining the basic fuel injection duration based on intake pipe pressure and the engine speed and correcting this basic duration by the signals from the sensors for optimum duration in accordance with the driving state of the vehicle.

Elements for correction are listed as follows.

(1) Voltage Correction

Duration of energization to the injector is extended based on a decrease of the battery voltage in order to correct a delay of injection due to the voltage drop of the battery.

(2) Engine Speed Correction

Duration of fuel injection is corrected based on the engine speed.

(3) Intake temperature correction

Difference between the densities of the air due to changes in the intake temperature is corrected.

(4) Air-Fuel Ratio Correction

Deviation from the target air-fuel ratio is corrected in each driving range.

(5) Feedback correction

The air-fuel ratio is corrected to be maintained substantially at the theoretical air-fuel ratio from the waste oxygen in the exhaust gas.

(6) Learning Correction

A base air-fuel ratio deviated by age deterioration is corrected to be maintained substantially at the theoretical air-fuel ratio.

(7) Warmed Engine Correction

The amount of fuel injection is increased with increase of the temperature of coolant for the cold engine, and the amount of correction is gradually decreased with increase of engine temperature.

(8) Atmospheric Pressure Correction

Deviation of the air-fuel ratio due to changes in atmospheric pressure is corrected. The atmospheric pressure is an estimated value calculated from the intake pipe pressure and the engine speed.

(9) Throttle Opening Angle Correction

The duration of the fuel injection is corrected based on the changes in throttle-opening angle.

(10) Correction of the Concentration of Purge

Changes of air-fuel ratio when the evaporated fuel gas is introduced or cut are corrected.

(11) Correction to Increase for Acceleration or Decrease for Deceleration

Upon detection of an acceleration or deceleration state of the vehicle, the amount of correction is increased for the acceleration of the vehicle to improve acceleration performance, while the amount of correction is decreased for deceleration to restrict exhaust gas emission and to improve fuel economy.

(12) Correction to Increase Immediately After the Start of the Engine

The amount of correction is increased immediately after start of the engine, and thereafter the amount of correction is gradually decreased for better operability.

In the embodiment of the present invention, the control means 56 control the amount of hydrogen gas being supplied to the hydrogen engine 3 based on the detection of the upstream-side exhaust gas sensor 54 as a feedback control so that the exhaust gas is at a predetermined lean mixture air-fuel ratio, and controls the amount of hydrogen gas supplied from the exhaust-side hydrogen gas injection device 32 to be less than that supplied to the hydrogen engine 3.

More particularly, the control means 56 performs the feedback control by which the amount of hydrogen gas supplied from the primary supply injector 37 of the intake-side hydrogen gas injection device 31 to the hydrogen engine 3 is controlled based on (he detected signal from the upstream-side exhaust gas sensor 54 so that the exhaust gas is at a predetermined lean air-fuel ratio (e.g. λ=1.05).

Also the control means 56 control hydrogen gas supplied from the secondary supply injector 39 of the exhaust-side hydrogen gas injection device 32 into the exhaust pipe 4 to be less than that supplied from the primary supply injector 37 of the intake-side hydrogen gas injection device 31 to the hydrogen engine 3.

In a basic hydrogen gas supply control, (he primary supply is greater in amount than the secondary supply, and the secondary supply is of a very small amount as compared to that of the primary supply. Even if the target of the air-fuel ratio control by the primary feedback control is directed toward lean mixture and therefore the secondary supply increases depending on the extent of leanness, the secondary supply may be relatively less than the primary supply.

Also, the control means 56 monitors the rate of reduction of the exhaust gas after having passed the catalyst with hydrogen gas supplied from the exhaust-side hydrogen gas injection device 32 based on the detection by the downstream-side exhaust gas sensor 55, and performs feedback control to correct the amount of hydrogen gas supply for increase or decrease thereof.

More particularly, the hydrogen engine 3 is a cylinder injection system for fuel supply.

Accordingly, when the fuel is injected over the compression stroke, the supply pressure (injection pressure) of hydrogen gas, i.e. fuel, from the primary supply injector 37 must overcome the internal pressure of the combustion chamber (cylinder).

On this account, the supply pressure must be decompressed by the pressure regulator 34 to be at relatively higher pressure than the atmospheric pressure and turbocharged pressure.

Incidentally, when the fuel is injected before the compression stroke, the output may be relatively less and the supply pressure (injection pressure) may be low in lean combustion.

At this moment, the supply pressure (injection pressure) from the secondary supply injector 39 that supplies secondary hydrogen gas into the exhaust pipe 4 must be at relatively higher pressure in consideration of the exhaust gas pressure of the turbocharged engine.

In case the supply pressure is high, it is preferable to not only diffuse but also concentrate hydrogen gas into the exhaust pipe 4, which further shortens the operational time of the secondary supply injector 39 whose operational time is inherently shorter than that of the primary supply injector 37.

In addition, some of (he excessive oxygen (O2) gases evaporate by the hydrogen (H2) gases in the catalyst 5, which reduces undue increase of the temperature of the catalyst.

Thereby, managing the catalyst 5 to be within the range of active temperatures is achievable.

Further, even if the hydrogen engine 3 comprises a multi-cylinder engine, the frequency of the secondary hydrogen gas supply may be less than that of (he number of the cylinders per crank rotation, which may cover injection timing being aimed at a particular cylinder.

It only matters that diffusion mixing proceeds during flowing in exhaust pipe 4 and the exhaust composition after having passed the catalyst is more purified in cooperation with retention in the exhaust system by the catalyst 5.

Still further, accurate feedback control using the upstream-side exhaust gas sensor 54 allows more precise correction by the downstream-side exhaust gas sensor 55.

In the above-mentioned exhaust gas purification system 1, hydrogen gas or the fuel is stored in the storage tank 2 at high pressure (several tens of MPa, for example 35-70 MPa).

As indicated by arrows in FIG. 2, hydrogen gas at high pressure in the storage tank 2 is then delivered through hydrogen gas supply passage 33 of hydrogen gas injection system 30 to the pressure regulator 34. The pressure regulator 34 decompresses hydrogen gas at high pressure (several tens of MPa, for example 35-70 MPa) to a lower pressure (several hundreds kPa, e.g. several atmospheric-pressures).

As indicated by the arrows in FIG. 2, the decompressed hydrogen gas is supplied through the primary supply passage 35 of the intake-side hydrogen gas injection device 31 to the delivery pipe 36 attached to the cylinder head 7, and is directly injected into the combustion chamber 10 through the primary supply injector 37 connected to the delivery pipe 36; hydrogen gas is then combusted to drive the hydrogen engine 3.

For lean-burn control, the upstream-side exhaust gas sensor 54 upstream of the catalyst 5 measures the ratio of oxygen for the feedback control, so that the amount of the fuel injection is controlled to achieve a target slightly lean air-fuel ratio (e.g. λ=1.05).

It is noted that, if the fuel is combusted at the air-fuel ratio of one or more (λ≧1), nitrogen in the mixture is oxidized to generate NOx as indicated below.

H₂+O₂+N₂→H₂O+NO_(x)+O₂+N₂

On this account, not only the primary hydrogen gas supply by the primary supply injector 37 of the intake-side hydrogen gas injection device 31, but also the secondary hydrogen gas supply by the secondary supply injector 39, and the exhaust-side hydrogen gas injection device 32 are performed.

Exhaust pipe 4 upstream of the catalyst 5 is provided with the secondary supply injector 39, and the pressure regulator 34 decompresses hydrogen gas at a pressure (several hundreds kPa) different from the pressure of hydrogen gas for the primary supply injector 37.

As indicated by the arrows in FIG. 2, some of the decompressed hydrogen gases are supplied to the secondary supply injector 39 through the secondary supply passage 38 of the exhaust-side hydrogen gas injection device 32, and are injected into the exhaust pipe 4 by the secondary supply injector 39. NO_(x), generated in the exhaust gas, is reduced on the catalyst 5 with hydrogen gas as a reducing agent to avoid emission of deleterious NO_(x) to the atmosphere.

Simultaneously, excessive O2 is oxidized on the catalyst with hydrogen gas and some of them are evaporated.

This is expressed by the equation as follows.

In this situation, the injection timing of the secondary supply injector 39 may be synchronized with one of the primary supply injectors 37, for example.

In order to control the amount of injection of hydrogen gas into the exhaust pipe 4 by the secondary supply injector 39 of the exhaust-side hydrogen gas injection device 32, the upstream-side exhaust gas sensor 54 monitors for the λ value; the excessive hydrogen gas that was not provided for reduction is controlled using the downstream-side exhaust gas sensor 55 downstream from the catalyst 5 so that the secondary supply injector 39 does not excessively inject hydrogen gas.

Open arrows in FIG. 2 indicate the flow of the intake air.

The intake air having passed the air cleaner 15 is directed through the intake pipe 16 to the compressor 25 of the turbocharger 23, and is compressed by the compressor 25 and reaches the intercooler 29.

The intake air cooled by the intercooler 29 is introduced through the throttle body 18 having the throttle valve 17 therein, the surge tank 19, and the intake manifold 20 to the combustion chamber 10 of the hydrogen engine 3.

In the throttle body 18, while the ISC valve 42 of the idle speed control 40 is opened, the intake air flows into the surge tank 19 while bypassing the throttle valve 17 through the bypass passage 41.

Hatch-patterned arrows in FIG. 2 indicate the flow of the exhaust gas.

The exhaust gas discharged from the combustion chamber 10 of the hydrogen engine 3 is introduced through the exhaust manifold 21 and the exhaust pipe 4 to the catalyst 5 of the catalyst converter 22 on the exhaust pipe 4.

Catalyst 5 concurrently decreases the harmful component HC (mainly due to the blowby gas), CO, and NO_(x) for purifying the exhaust gas to discharge outside.

Next, the operation of the exhaust gas purification system 1 of the hydrogen engine 3 is explained with reference to the control flowchart in FIG. 1.

A control program for exhaust gas purification system 1 starts in step 102, and a determination is made in step 104 as to whether a condition of λ feedback, such as that where the engine is warm, is ON.

If the determination in step 104 is “NO”, then the determination in step 104 is repeated until the λ feedback condition is ON.

If the determination in step 104 is “YES”, then a process for starting the λ feedback operation is performed instep 106.

In this λ feedback operation in step 106, the primary supply injector 37 and the upstream-side exhaust gas sensor 54 that is the λ sensor operates for an ordinary PI control.

The PI control is generally called proportional and integral operation in which integral operation is applied to the proportional operation.

After the step 106, a closed loop process (sub loop) is performed in step 108.

In the closed loop (sub loop) in step 108, the signals detected by the downstream-side exhaust gas sensor 55 as the H2 sensor are received, and a determination is made in step 110 as to whether the concentration of H₂ is greater than or equal to a constant CH₂ (concentration of H₂≧CH₂).

If the determination in step 110 is “YES”, then a process is performed in step 112 which decreases a signal for valve opening time of the secondary supply injector 39 by an increasing/decreasing value TH2 of the injector operation time (also referred to as “a constant of target concentration feedback control”).

If the determination in step 110 is “NO” (i.e. concentration of H₂<CH₂), then a process is performed in step 114 which increases the signal for valve opening time of the secondary supply injector 39 is increased by the value TH2 for the injector opening time.

After the steps 112 and 114, the process returns to the step 110 wherein the determination is made as to whether the concentration of H₂ is greater than or equal to the constant of the target H₂ concentration.

After the process in step 108, a determination is made as to whether the λ feedback condition is “OFF”.

If the determination in step 116 is “NO”, then the process returns to step 106 for determination of whether the λ feedback condition is “ON”.

If the determination in step 116 is “YES”, then the program ends in step 118.

As thus described the present invention provides an exhaust gas purification system 1 for the hydrogen engine 3, having the hydrogen storage tank 2 for storing hydrogen at high pressure, the hydrogen engine 3 for combusting hydrogen supplied from the hydrogen storage tank 2, and the catalyst 5 provided in the exhaust pipe 4 of the hydrogen engine 3. Exhaust gas purification system 1 is capable of supplying hydrogen gas through the exhaust pipe 4 during operation of the hydrogen engine 3. Upstream-side exhaust gas sensor 54 is provided on the exhaust pipe 4 upstream of the catalyst 5 for detecting the exhaust composition. Exhaust-side hydrogen gas injection device 32 is provided upstream of the upstream-side exhaust gas sensor 54 for injecting hydrogen gas into the exhaust pipe 4. Control means 56 control the amount of hydrogen gas for supplying to the hydrogen engine 3 based on the detection of the upstream-side exhaust gas sensor 54 as a feedback control so as to achieve the exhaust gas having a predetermined lean air-fuel ratio, and controls the amount of hydrogen gas supplied from the exhaust-side hydrogen gas injection device 54 to be less than that supplied to the hydrogen engine 3.

Accordingly, the combustion of the gas is maintained on average in a lean mixture state, which considerably reduces the emission of the exhaust element, in particular NH3.

Resultant increasing NOx can be selectively reduced on the catalyst with the addition of the supplied secondary hydrogen gas, which greatly maintains overall purification performance of the catalyst.

Further, the consumption of the supplied secondary hydrogen gas can be decreased.

Also, the control means 56 monitors the rate of reduction of the exhaust gas after having passed the catalyst with hydrogen gas supplied from the exhaust-side hydrogen gas injection device 32 based on the detection by the downstream-side exhaust gas sensor 55, and performs feedback control to correct the amount of hydrogen gas supply for increase or decrease thereof.

Thereby, the consumption of the supplied secondary hydrogen gas can be considerably decreased, improving the exhaust gas purification performance and fuel consumption.

The present invention is not limited to above-mentioned embodiment and changes and modifications can be made.

According to the embodiment of the present invention, the downstream-side exhaust gas sensor acts as the hydrogen (H₂) sensor to control excessive hydrogen gas that was not provided for reduction, and the amount of fuel injection from the secondary supply injector is controlled so as not to inject in excess. Instead of the hydrogen (H₂) sensor, the downstream-side exhaust gas sensor may act as the oxygen (O₂) sensor to monitor the amount of O₂ in the exhaust gas for feedback control of the amount of hydrogen gas injected from the secondary supply injector into the exhaust pipe. Alternatively, a NO_(x) sensor may be utilized.

Where the oxygen (O₂) sensor is utilized instead of the hydrogen (H₂) sensor, a ratio of reduction with hydrogen gas by the catalyst may be determined empirically, so that the oxygen sensor may be utilized as an equivalent.

Also, in ordinary lean burn gasoline engines and diesel engines, reduction of NOx can be achieved when the engine has a structure wherein the hydrogen storage tank is mounted for reduction and hydrogen gas in the tank is injected into the exhaust gas.

Further, in a certain engine driving range based on engine load, the amount of hydrogen gas supplied from the secondary supply injector as the secondary supply may be kept within limits such that the total amount of injection is within a lean side from the stoichiometric mixture ratio, based on the amount of supply from the primary supply injector as the primary supply.

Still further, according to the above-mentioned embodiment, the pressure regulator at once reduces the pressure in one step, decompressing hydrogen gas at high pressure (several tens of MPa, for example 35-70 MPa) to the lower pressure (several hundreds kPa, e.g. several atmospheric-pressures). Instead, multiple-decompression may be allowable which decompresses in more than two stages, such as primary decompression and secondary decompression.

Thereby, multiple-decompression permits the decompression of hydrogen gas progressively and reliably.

Moreover, as the catalyst, a catalyst for “bi-fuel” such as evaporative fuel-gasoline and a catalyst for an FFV (flexible fuel vehicle) may also be utilized.

EXPLANATION OF REFERENCE NUMERALS

1 exhaust gas purification system;

2 hydrogen storage tank;

3 hydrogen engine;

4 exhaust pipe;

5 catalyst;

10 combustion chamber;

16 intake pipe;

20 intake manifold;

21 exhaust manifold;

22 catalytic converter;

23 turbocharger;

29 intercooler;

30 hydrogen as injection system;

31 intake-side hydrogen gas injection device;

32 exhaust-side hydrogen gas injection device;

33 hydrogen gas supply passage;

34 pressure regulator;

35 primary supply passage;

37 primary supply injector;

38 secondary supply passage;

39 secondary supply injector;

40 idle speed controller;

42 ISC valve;

47 fuel pressure sensor;

49 engine coolant temperature;

50 throttle sensor;

52 intake pressure sensor;

53 intake temperature sensor;

54 upstream-side exhaust gas sensor (“air-fuel sensor” or “λ sensor”);

55 downstream-side exhaust gas sensor (“H2 sensor”);

56 control means (“ECM”); and

57 crank angle sensor. 

1. An exhaust gas purification system for a hydrogen engine, having a hydrogen storage tank for storing hydrogen at high pressure, a hydrogen engine for combusting hydrogen supplied from the hydrogen storage tank, and a catalyst provided in an exhaust pipe of the hydrogen engine, the exhaust gas purification system being capable of supplying hydrogen gas into the exhaust pipe during operation of the hydrogen engine, comprising: an upstream-side exhaust gas sensor disposed on the exhaust pipe upstream of the catalyst for detecting the exhaust gas composition; an exhaust-side hydrogen gas injection device disposed upstream of the upstream-side exhaust gas sensor for injecting hydrogen gas into the exhaust pipe; and control means which control the amount of hydrogen gas for supply to the hydrogen engine based on the detection by the upstream-side exhaust gas sensor as a feedback control so as to obtain an exhaust gas having a predetermined lean air-fuel ratio, and which control the amount of hydrogen gas supplied from the exhaust-side hydrogen gas injection device to be less than that supplied to the hydrogen engine.
 2. The exhaust gas purification system for a hydrogen engine as defined in claim 1, wherein a downstream-side exhaust gas sensor for detecting exhaust gas composition is provided in the exhaust pipe downstream from the catalyst, the control means monitors a rate of reduction of the exhaust gas after having passed (he catalyst with hydrogen gas supplied from the exhaust-side hydrogen gas injection device, based on the detection by the downstream-side exhaust gas sensor, and perform a feedback control to correct the amount of hydrogen gas supply for increase or decrease thereof. 